Values 'a', 'A', and 'H' may be followed by a repeat value. A repeat value
of '*' will cause the remainder of the bytes in values to be placed
in the last element.

'/' allows packing and unpacking of a sequence of values where the packed
structure contains a packed item count followed by the packed items themselves.

If there are more template values or if the repeat count of a field
or a group is larger than what the remainder ... allows, unpack
returns NULL. If ... is longer than what is described by
template, the rest is ignored.